The present invention relates to an elastomeric bearing, and more particularly to a heatable elastomeric bearing with integral heating elements which facilitates cold weather operation.
Elastomeric bearings are used in many applications. Elastomeric bearings typically include alternating layers of elastomeric material and metallic or composite shims. These elastomeric bearings often replace non-lubricated or self-lubricated bearings such as Teflon fabric lined bearings. Typical aerospace elastomeric bearing applications include spherical rod end bearings for pitch control rods and dampers, spherical blade retention bearings for fully articulated rotors, and cylindrical bearings for semi-articulated rotors and fluid-elastic damper seals.
Cold temperatures may reduce the service life and static strength of elastomeric bearings as the elastomeric material therein may be relatively sensitive to such cold temperatures. The elastomeric bearings may also stiffen when exposed to cold temperatures. This may result in an increased spring rate that transfers increased loads to the mating components that may reduce fatigue life of the mating components. An increased spring rate may also result in dynamic behavior alteration of systems which utilize the elastomeric bearings.
In rotary-wing aircraft operations, it is common practice to use a warm-up procedure to soften the elastomeric bearings prior to rotor start-up. This typically involves pilot movements of the collective, cyclic, and yaw controls, with gradually increasing amplitude and frequency. This procedure is time consuming and is difficult due to the bulky cold weather gear often worn by the pilots and/or ground crew. The procedure is also less effective on certain elastomeric bearings that do not move significantly in response to control movement (i.e. damper bearings).
For rotary wing aircraft which utilizes a servo flap control system, a torsional stiffness constraint is typically imposed on the spherical elastomeric blade retention bearing. Variations in this spring rate due to cold temperature may affect stability and dynamic characteristics of the rotor. Further complicating pre-flight procedures of aircraft with servo flap control, stick movement will not generate movement of the blade retention bearings. For fluid/elastic damper systems, current pre-flight warm-up procedures may be ineffective at warming the elastomeric cylindrical “seals”, resulting in high initial spring rates, with increased loadings on the damper retention hardware. This may result in the aforementioned dynamic and service life concerns.
Accordingly, it is desirable to provide a system and method to warm-up elastomeric bearings prior to rotor start-up.